Apparatus and method for modulating photon output of a quantum dot light emitting device

ABSTRACT

An apparatus is provided for modulating the photon output of a plurality of free standing quantum dots. The apparatus comprises a first electron injection layer ( 210, 310, 410 ) disposed between a first electrode ( 212, 312, 412 ) and a layer ( 208, 308, 408 ) of the plurality of free standing quantum dots. A hole transport layer ( 206, 306, 406 ) is disposed between the layer ( 208, 308, 408 ) of the plurality of quantum dots and a second electrode ( 204, 304, 404 ). A light source ( 224, 324, 424 ) is disposed so as to apply light to the layer ( 208, 308, 408 ) of the plurality of free standing quantum dots. The photon output of the layer ( 208, 308, 408 ) of the plurality of free standing quantum dots is modulated by applying a voltage to the first and second electrodes ( 212, 312, 412, 204, 304, 404 ). Electrons excited to a higher energy state within layer ( 208, 308, 408 ) of the free standing quantum dots by the light source ( 224, 324, 424 ) are prevented from returning to a lower state by electrons from the electric field of the applied voltage, and therefore the free standing quantum dots are prevented from emitting a photon. The voltage source ( 216, 316, 416 ) may be modulated to vary the photon output.

FIELD OF THE INVENTION

The present invention generally relates to light emitting devices andmore particularly to a method for adjusting the photon output of quantumdots.

BACKGROUND OF THE INVENTION

Free standing quantum dots (FSQDs) are semiconductor nanocrystalliteswhose radii are smaller than the bulk exciton Bohr radius and constitutea class of materials intermediate between molecular and bulk forms ofmatter. FSQDs are known for the unique properties that they possess as aresult of both their small size and their high surface area. Forexample, FSQDs typically have larger absorption cross-sections thancomparable organic dyes, higher quantum yields, better chemical andphoto-chemical stability, narrower and more symmetric emission spectra,and a larger Stokes shift. Furthermore, the absorption and emissionproperties vary with the particle size and can be systematicallytailored. It has been found that a Cadmium Selenium (CdSe) quantum dot,for example, can emit light in any monochromatic, visible color, wherethe particular color characteristic of that dot is dependent on the sizeof the quantum dot.

FSQDs are easily incorporated into or on other materials such aspolymers and polymer composites because FSQDs are highly soluble andhave little degradation over time. These properties allow FSQD polymersand polymer composites to provide very bright displays, returning almost100% quantum yield.

Applications for FSQD polymers and polymer composites include point ofpurchase and point of sale posters, mobile device housings or logos,segmented displays, including ultraviolet (UV) and infrared (IR)displays, absorbers for UV and IR sensors or detectors, and lightemitting diodes (LEDs). Although the visible advantages inherent to FSQDpolymers and polymer composites are attractive, control of the output(light intensity) is problematic.

Accordingly, it is desirable to provide an apparatus and method ofmodulating the photon output of a FSQD light emitting device.Furthermore, other desirable features and characteristics of the presentinvention will become apparent from the subsequent detailed descriptionof the invention and the appended claims, taken in conjunction with theaccompanying drawings and this background of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a schematic partial cross section illustrating a previouslyknown free standing quantum dot display;

FIG. 2 is a schematic partial cross section illustrating a firstexemplary embodiment;

FIG. 3 is a partial cross section of a second exemplary embodiment;

FIG. 4 is a partial cross section of a third exemplary embodiment; and

FIG. 5 is a front view of a portable electronic device including adisplay suitable for use with the exemplary embodiment; and

FIG. 6 is a block diagram illustrating circuitry for implementingvarious exemplary embodiments on the portable electronic device of FIG.1.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

Quantum Dots (QDs), also known as nanocrystals or Freestanding QuantumDots (FSQD), are semiconductors composed of periodic groups of II-VI,III-V, or IV-VI materials, for example, CdS, CdSe, CdTe, ZnS, ZnSe,ZnTe, GaAs, GaP, GaAs, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs,AlP, AlSb. Alternative FSQDs materials that may be used include but arenot limited to tertiary microcrystals such as InGaP, which emits in theyellow to red wavelengths (depending on the size) and ZnSeTe, ZnCdS,ZnCdSe, and CdSeS which emits from blue to green wavelengths. Multi-corestructures are also possible such as ZnSe/ZnXS/ZnS, are also possiblewhere X represents Ag, Sr, Te, Cu, or Mn. The inner most core is made ofZnSe, followed by the second core layer of ZnXS, completed by anexternal shell made of ZnS.

FSQDs range in size from 2-10 nanometers in diameter (approximately10²-10⁷ total number of atoms). At these scales, FSQDs have size-tunableband gaps, in other words there spectral emission depends upon size.Whereas, at the bulk scale, emission depends solely on the compositionof matter. Other advantages of FSQDs include high photoluminescencequantum efficiencies, good thermal and photo-stability, narrow emissionline widths (atom-like spectral emission), and compatibility withsolution processing. FSQDs are manufactured conventionally by usingcolloidal solution chemistry.

FSQDs may be synthesized with a wider band gap outer shell, comprisingfor example ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs,GaN, GaP, GaAs, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InSb, AlAs,AlN, AlP, AlSb. The shell surrounds the core FSQDs and results in asignificant increase in the quantum yield. Capping the FSQDs with ashell reduces non-radiative recombination and results in brighteremission. The surface of FSQDs without a shell has both free electronsin addition to crystal defects. Both of these characteristics tend toreduce quantum yield by allowing for non-radiative electron energytransitions at the surface. The addition of a shell reduces theopportunities for these non-radiative transitions by giving conductionband electrons an increased probability of directly relaxing to thevalence band. The shell also neutralizes the effects of many types ofsurface defects. The FSQDs are more thermally stable than organicphosphors since UV light will not chemically breakdown FSQDs. Theexterior shell can also serve as an anchor point for chemical bonds thatcan be used to modify and functionalize the surface.

Due to their small size, typically on the order of 10 nanometers orsmaller, the FSQDs have larger band gaps relative to a bulk material. Itis noted that the smaller the FSQDs, the higher the band gap. Therefore,when impacted by a photon (emissive electron-hole pair recombination),the smaller the diameter of the FSQDs, the shorter the wavelength oflight will be released. Discontinuities and crystal defects on thesurface of the FSQD result in non-radiative recombination of theelectron-hole pairs that lead to reduced or completely quenched emissionof the FSQD. An overcoating shell, e.g., ZnS, having a thickness, e.g.,of up to 5 monolayers and higher band gap compared to the core's bandgap is optionally provided around the FSQDs core to reduce the surfacedefects and prevent this lower emission efficiency. The band gap of theshell material should be larger than that of the FSQDs to maintain theenergy level of the FSQDs. Capping ligands (molecules) on the outersurface of the shell allow the FSQDs to remain in the colloidalsuspension while being grown to the desired size. The FSQDs may then beplaced within the display by a printing process, for example.Additionally, a light (radiation) source (preferably a ultra violet (UV)source) is disposed to selectively provide photons to strike the FSQDs,thereby causing the FSQDs to emit a photon at a frequency comprising thespecific color as determined by the size tunable band gap of the FSQDs.

A layer comprising a plurality of FSQDs disposed between an electrontransport layer (or hole blocking layer) and a hole transport layer,when exposed to photons from a light source, will emit light having afrequency determined by the size of the FSQDs as long as the absorbedwavelength is shorter than the emitted wavelength. Application of avoltage potential across the structure will create a saturation of alarger population of electron or hole pairs (excitons) that quenches theemission of the photonicly excited emission. The light from the lightsource excites electrons from the ground state of the FSQDs into ahigher electric energy/vibration state. The applied electric field ofthe voltage potential injects the electrons into free holes (includingthose in the ground energy state), prohibiting the electrons in a higherenergy state to return to the ground energy state. Since photon emissiononly occurs when the electron relaxes into the ground-level energystate, photon emission is reduced. The level of photon emission from theFSQDs may be controlled by varying the voltage potential.

The exemplary embodiments described herein may be fabricated using knownlithographic processes as follows. The fabrication of integratedcircuits, microelectronic devices, micro electro mechanical devices,microfluidic devices, and photonic devices, involves the creation ofseveral layers of materials that interact in some fashion. One or moreof these layers may be patterned so various regions of the layer havedifferent electrical or other characteristics, which may beinterconnected within the layer or to other layers to create electricalcomponents and circuits. These regions may be created by selectivelyintroducing or removing various materials. The patterns that define suchregions are often created by lithographic processes. For example, alayer of photoresist material is applied onto a layer overlying a wafersubstrate. A photomask (containing clear and opaque areas) is used toselectively expose this photoresist material by a form of radiation,such as ultraviolet light, electrons, or x-rays. Either the photoresistmaterial exposed to the radiation, or that not exposed to the radiation,is removed by the application of a developer. An etch may then beapplied to the layer not protected by the remaining resist, and when theresist is removed, the layer overlying the substrate is patterned.Alternatively, an additive process could also be used, e.g., building astructure using the photoresist as a template.

Though various lithography processes, e.g., photolithography, electronbeam lithography, and imprint lithography, ink jet printing, may be usedto fabricate the light emitting device, a printing process is preferred.In the printing process, the FSQD ink in liquid form is printed indesired locations on the substrate. Ink compositions typically comprisefour elements: 1) functional element, 2) binder, 3) solvent, and 4)additive. Graphic arts inks and functional inks are differentiated bythe nature of the functional element, i.e. the emissive quantum dot. Thebinder, solvent and additives, together, are commonly referred to as thecarrier which is formulated for a specific printing technology e.g.tailored rheology. The function of the carrier is the same for graphicarts and printed electronics: dispersion of functional elements,viscosity and surface tension modification, etc. One skilled in the artwill appreciate that an expanded color range can be obtained by usingmore than three quantum dot inks, with each ink having a different meanquantum dot size. A variety of printing techniques, for example, Flexo,Gravure, Screen, inkjet may be used. The Halftone method, for example,allows the full color range to be realized in actual printing.

Referring to FIG. 1, a cross sectional view of a known light emittingdevice 100 includes a first electrode 104 formed on a substrate 102. Thesubstrate is formed of a transparent, sturdy, thin material such asglass, but may comprise a flexible polymer such as polyethyleneterephthalate (PET) or polyethylene naphthalate (PEN). A hole transportlayer 106 is formed on the first electrode 104, for example, indium tinoxide (ITO) or poly-3,4-ethylenedioxthiophene (PEDOT). A layer 108 of aplurality of FSQDs is formed on the hole transport layer 106. Note thatthe hole and transport layers could also be made of inorganic materials.An electron injection layer 110 and a second electrode 112 are thenformed over the layer 108. The substrate 102 typically comprises atransparent material. The first and second electrodes 104, 112 functionas an anode and a cathode respectively. The first electrode 104 istransparent while the second electrode 112, is typically opaque.

When the layer 108 of the plurality of FSQDs are impacted with lighthaving a wavelength shorter that which would be emitted by the FSQDs, anelectron in each of the FSQDs so impacted is excited to a higher level.When the electron falls back to its ground state, a photon is emittedhaving a wavelength determined by the diameter of the FSQD.

Referring to FIG. 2 and in accordance with a first exemplary embodiment,a cross sectional view of a light emitting device 200 includes a lightsource 224 deposited on a substrate 212. The light source 224 is coupledto a voltage source 226 through switch 228 for selectively activatingthe light source 224. It is understood that the light source 224 may bepositioned in any location wherein its output may be applied to theFSQDs, and may comprises any frequency below that provided as outputfrom the FSQDs, but preferably comprises ultraviolet (UV) radiation. Afirst electrode 204 is formed on a light source 224. A hole transportlayer 206 is formed on the first electrode 204, but may alternativelycomprise an electron blocking layer. A layer 208 of a plurality of FSQDsis formed on the hole transport layer 206. An electron injection layer210 and a second electrode 212 are then formed over the layer 208. Thesubstrate 202 preferably comprises any transparent material, but maycomprise, for example, glass, ceramic, insulated metal, polymers, andpolymer composites. The first electrode 204 (anode) comprises atransparent material, preferably indium tin oxide, and has a workfunction ranging from 4.3-5.1 eV. The second electrode 212 (cathode) hasa lower work function such as Al, Ca, Li, Cs. It is recognized that thesubstrate 202 may comprise a rigid structure or be flexible, andalthough is disposed adjacent the first electrode 204, it mayalternatively be disposed adjacent the second electrode 212 A voltagesource 214 is coupled across the first and second electrodes 204, 212 ofthe light source 200. A switch 216 may be coupled between the voltagesource 214 and one of the first and second electrodes 204, 212 toselectively apply the voltage to the light source 200. Furthermore, theswitch 216 may be adjusted to vary the amount of potential provided bythe voltage source 214.

In operation, when the layer 208 of the plurality of FSQDs are impactedwith light having a wavelength shorter that which would be emitted bythe FSQDs, an electron in each of the FSQDs so impacted is excited to ahigher level. When the electron falls back to its ground state, a photonis emitted having a wavelength determined by the size of the FSQD.However, the potential supplied to the FSQDs by the voltage source 214injects electrons into free holes (including those in the ground energystate) of the FSQDs, prohibiting the electrons in a higher energy stateto return to the ground energy state. Oversaturation occurs because moreelectrons are injected than there are holes. Since photon emission onlyoccurs when the electron relaxes into the ground-level energy state,photon emission is reduced. The level of photon emission from the FSQDsmay be controlled by varying the voltage potential of the voltage source214 by the switch 216.

Referring to FIG. 3 and in accordance with a second exemplaryembodiment, a cross sectional view includes three segments, or pixels,301, 301′, 301″ of a light emitting device 300. The segments 301, 301′,301″ are fabricated, or disposed, contiguous to one another in a side byside arrangement. Each of the segments 301, 301′, 301″ are fabricatedsimilar to the device 200, with each similar element identified by anumerical reference incremented by 100. Similar elements in each of thesegments 301, 301′, 301″ are identified with a number in segment 301, aprime of the number in the segment 301′, and a double prime in thesegment 301″. It should be understood that although three voltagesources 316, 316′, 316″ are shown, a single voltage source mayalternatively supply a potential to each of the three segments 301,301′, 301″ through the switches 316, 316′, 316″. This side by sidearrangement allows for each of the segments 300, 301′, 301″ to bemodulated independently by controlling each of the switches 316, 316′,316″. This electronic device 300 may be used, for example, on a postersuch as in advertising or on a housing of an electronic device such as acellular phone. By adjusting the switches 16, 316′, 316″ (potential),the light intensity of each segment 301, 301′, 301″ may be varied tocreate lighter and darker areas. This difference in intensity may beused to provide information such as text or to illuminate icons. It mayalso provide an effect of a moving light by modulating the intensity ofthe three segments 301, 301′, 301″ in succession.

Furthermore, since the color of the light emitted by the FSQDs dependson the size of the FSQDs, by fabricating the FSQDs 308, 308′, 308″ withdifferent diameters, the color presented on the poster or housing may beselected by activating the appropriate switch 314, 314′, 314″.

A cross sectional view of a third exemplary embodiment is shown in FIG.4 three tiers 401, 401′, 401″ stacked in a single pixel 400. Each of thetiers 401, 401′, 401″ are fabricated similar to the device 200 and arefastened together, with each similar element identified by a numericalreference incremented by 200 from the numbers in FIG. 2. One method offastening is an index-matched optical adhesive 418. Similar elements ineach of the tiers 401, 401′, 401″ are identified with a number in tier401, a prime of the number in the tier 401′, and a double prime in thetier 401″. It should be understood that although three voltage sources416, 416′, 416″ are shown, a single voltage source may alternativelysupply a potential to each of the three segments 401, 401′, 401″ throughthe switches 416, 416′, 416″. This stacked embodiment allows for each ofthe tiers 400, 401′, 401″ to be modulated independently by controllingeach of the switches 416, 416′, 416″. It is noted that although threelight sources 424, 424′, 424″ are shown, a different embodiment maycomprise only a single light source, e.g., light source 424. It isfurther noted that in this exemplary embodiment, the electrodes 412,412′, 412″ are transparent (though in other embodiments they may beopaque). Transparency may be accomplished, for example, by making theelectrode 412, 412′, 412″ very thin or by using a redox reaction tomodify the surface of the material, e.g., indium tin oxide, to decreasethe work function down to 3.7 eV.

FSQDs 408, 408′, 408″ are distributed at a density within a transparentpolymer material 409, 409′, 409″, respectively, that permits photonstherethrough. A voltage is applied to the UV light sources 424, 424′,424″, causing photons to be emitted, striking the FSQDs 408, 408′, 408″.Light (photons) are then emitted from the QDs having a predeterminedcolor (frequency) depending on the size of the FSQDs. The density of theFSQDs 408, 408′, 408″ are such that photons from the UV light sources424, 424′, 424″ will be sufficient to strike the FSQDs 408, 408′, 408″in each of the tiers 401, 401′, 401″, with some of the UV photonspassing through the lower tiers 401, 401′ to reach the upper tiers 401′,401′. The colors provided by the FSQDs 408, 408′, 408″ are disposed suchthat the longer wavelength colors are closest to the anode, e.g.,cathode-red-green-blue-anode. This prevents the photons (red) emittingfrom the FSQDs 408 from activating the FSQDs 408′ that emit green (redphotons will not excite green photons, however green photons will excitethe red ones).

Although only one pixel 400 is shown, it should be understood that alarge number of pixels may be formed and subdivided into regions withvarious shapes to permit different areas to display different colors orinformation.

In operation, when a desired color and/or pattern (including informationsuch as text) is determined, signals are sent to each of the switches416, 416′, 416″. As the voltage is applied to the respective tier 401,401′, 401″, the electrons enter the FSQDs 408, 408′, 408″ within thatrespective tier 401, 401′, 401″, thereby preventing photons from beingemitted. Consequently, different shades of colors (grayscales) areobtained by modulating the applied voltage level. Therefore, by applyingthe proper signals to each of the tiers 401, 401′, 401″, the desiredcolor is achieved.

Referring to FIG. 5, a portable electronic device 510 comprises adisplay 512, a control panel 514, and a speaker 516 encased in a housing520. Some portable electronic devices 510, e.g., a cell phone, mayinclude other elements such as an antenna, a microphone, and a camera(none shown). In the exemplary embodiments described herein, the display512 comprises a FSQD display technology. The exemplary embodiment maycomprise any type of electronic device, for example, a PDA, a mobilecommunication device, and gaming devices. Furthermore, while thepreferred exemplary embodiment of a portable electronic device isdescribed as a mobile communication device, other embodiments areenvisioned, such as flat panel advertising screens, point of purchaseand point of sale posters, mobile device housings or logos, segmenteddisplays, including infrared displays, absorbers for infrared sensors ordetectors, and light emitting diodes (LEDs).

Referring to FIG. 6, a block diagram of a portable electronic device 610such as a cellular phone, in accordance with the exemplary embodiment isdepicted. Though the exemplary embodiment is a cellular phone, thedisplay described herein may be used with any electronic device in whichinformation, colors, or patterns are to be presented. The portableelectronic device 610 includes an antenna 612 for receiving andtransmitting radio frequency (RF) signals. A receive/transmit switch 614selectively couples the antenna 612 to receiver circuitry 616 andtransmitter circuitry 618 in a manner familiar to those skilled in theart. The receiver circuitry 616 demodulates and decodes the RF signalsto derive information therefrom and is coupled to a controller 620 forproviding the decoded information thereto for utilization thereby inaccordance with the function(s) of the portable communication device610. The controller 620 also provides information to the transmittercircuitry 618 for encoding and modulating information into RF signalsfor transmission from the antenna 612. As is well-known in the art, thecontroller 620 is typically coupled to a memory device 622 and a userinterface 114 to perform the functions of the portable electronic device610. Power control circuitry 626 is coupled to the components of theportable communication device 610, such as the controller 620, thereceiver circuitry 616, the transmitter circuitry 618 and/or the userinterface 114, to provide appropriate operational voltage and current tothose components. The user interface 114 includes a microphone 628, aspeaker 116 and one or more key inputs 632, including a keypad. The userinterface 114 may also include a display 112 which could include touchscreen inputs. The display 112 is coupled to the controller 620 by theconductor 636 for selective application of voltages in some of theexemplary embodiments described above.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

The invention claimed is:
 1. A method for modulating intensity of photonoutput from free standing quantum dots, the method comprising: applyingradiation including a wavelength shorter than a first predeterminedwavelength to a first plurality of free standing quantum dots having asize selected for emitting light having the first predeterminedwavelength to create a first photon output including the firstpredetermined wavelength; and modulating the intensity of the firstphoton output created by the application of radiation to the firstplurality of free standing quantum dots by adjusting the magnitude of afirst plurality of electrons injected into the first plurality of freestanding quantum dots which are adjacent an electron transport layer ora hole transport layer and disposed between two electrodes to provide adesired first photon output intensity, wherein the application ofradiation to the first plurality of free standing quantum dots and theinjection of the first plurality of electrons into the first pluralityof free standing quantum dots are separately controlled.
 2. The methodof claim 1 further comprising: applying radiation including a wavelengthshorter than a second predetermined wavelength to a second plurality offree standing quantum dots having a size selected for emitting lighthaving the second predetermined wavelength disposed contiguous to thefirst plurality of free standing quantum dots to create a second photonoutput including the second predetermined wavelength; and modulating theintensity of the second photon output created by the application ofradiation to the second plurality of free standing quantum dots byadjusting the magnitude of a second plurality of electrons injected intothe second plurality of free standing quantum dots to provide a desiredsecond photon output intensity, wherein the application of radiation tothe second plurality of free standing quantum dots and the injection ofthe second plurality of electrons into the second plurality of freestanding quantum dots are separately controlled.
 3. The method of claim2 further comprising: applying radiation including a wavelength shorterthan a third predetermined wavelength to a third plurality of freestanding quantum dots having a size selected for emitting light havingthe third predetermined wavelength disposed contiguous to the secondplurality of free standing quantum dots to create a third photon outputincluding the third predetermined wavelength; and modulating theintensity of the third photon output created by the application ofradiation to the third plurality of free standing quantum dots byadjusting the magnitude of a third plurality of electrons injected intothe third plurality of free standing quantum dots to provide a desiredthird photon output intensity, wherein the application of radiation tothe third plurality of free standing quantum dots and the injection ofthe third plurality of electrons into the second plurality of freestanding quantum dots are separately controlled.
 4. The method of claim3 wherein the first, second, and third plurality of free standingquantum dots are stacked and each of the first plurality of freestanding quantum dots comprise a first diameter, each of the secondplurality of free standing quantum dots comprise a second diameter, andeach of the third plurality of free standing quantum dots comprise athird diameter, the method further comprising: providing a color outputby injecting the first, second, and third plurality of electrons.
 5. Themethod of claim 3 wherein the second plurality of free standing quantumdots are disposed between and adjacent to the first and third pluralityof free standing quantum dots, the method further comprising: providingadjacent areas of light by selectively injecting the first, second, andthird plurality of electrons.
 6. A method for modulating intensity ofphoton output from free standing quantum dots, the method comprising:applying radiation including a wavelength shorter than a firstpredetermined wavelength to a first plurality of free standing quantumdots having a size selected for emitting light having the firstpredetermined wavelength to create a first photon output; and modulatingthe intensity of the first photon output by adjusting the magnitude of afirst plurality of electrons injected into the first plurality of freestanding quantum dots, wherein the modulating step comprises controllingvoltage potential applied across an electron injection layer and a holetransport layer disposed on opposed sides of the first plurality of freestanding quantum dots, wherein the application of radiation to the firstplurality of free standing quantum dots and the injection of electronsinto the first plurality of free standing quantum dots are separatelycontrolled.
 7. A method of modulating intensity of the photon output ofa plurality of free standing quantum dots, comprising; applying lightincluding a wavelength shorter than a first predetermined wavelength toa first layer of free standing quantum dots having a size selected foremitting light having the first predetermined wavelength to create afirst photon output including the first predetermined wavelength; andcontrolling a first voltage applied to a first electron injection layerand a first hole transport layer disposed on opposed sides of the firstlayer comprising the plurality of free standing quantum dots to modulatethe intensity of the first photon output, wherein the application oflight to the first layer of free standing quantum dots and theapplication of the first voltage are separately controlled.
 8. Themethod of claim 7 further comprising varying the first voltage to obtaina desired first photon output.
 9. The method of claim 7 furthercomprising: applying light including a wavelength shorter than a secondpredetermined wavelength to a second layer of free standing quantum dotshaving a size selected for emitting light having the secondpredetermined wavelength to create a second photon output including thesecond predetermined wavelength; and controlling a second voltageapplied to a second electron injection layer and a second hole transportlayer disposed on opposed sides of the second layer of free standingquantum dots to modulate the intensity of the second photon output,wherein the first and second layers are disposed adjacent to oneanother.
 10. The method of claim 9 further comprising: applying lightincluding a wavelength shorter than a third predetermined wavelength toa third layer of free standing quantum dots having a size selected foremitting light having the third predetermined wavelength to create athird photon output including the third predetermined wavelength; andcontrolling a third voltage applied to a third electron injection layerand a third hole transport layer disposed on opposed sides of the thirdlayer of free standing quantum dots to modulate the intensity of thethird photon output, wherein the second and third layers are disposedadjacent to one another.
 11. The method of claim 10 wherein the first,second, and third layers of free standing quantum dots are stacked andeach of the first layer of plurality of free standing quantum dotscomprise a first diameter, each of the second layer of plurality of freestanding quantum dots comprise a second diameter, and each of the thirdlayer of plurality of free standing quantum dots comprise a thirddiameter, the method further comprising: providing a color output byselectively applying the first, second, and third plurality voltages.12. A method for controlling the level of photon output from freestanding quantum dots, the method comprising: providing a structureincluding a layer comprising a first plurality of free standing quantumdots having a size selected for emitting light having a firstpredetermined wavelength, the layer being disposed between a holetransport layer and an electron transport layer, a first electrodedisposed over the hole transport layer, and a second electrode disposedover the electron transport layer; exposing the layer to photons from alight source, the photons including photons having a wavelength shorterthan the first predetermined wavelength, to create a first photon outputincluding photons having the first predetermined wavelength; applying avoltage potential across the structure to inject electrons into theplurality of free standing quantum dots; and controlling the level ofthe first photon output by varying the voltage potential applied acrossthe structure, wherein the application of voltage potential across thestructure is independently modulated.